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Masatoshi Koshiba

Masatoshi Koshiba(小柴 昌俊Koshiba Masatoshi) (born on September 19, 1926 in Toyohashi, Aichi) is a Japanese physicist. He jointly won the Nobel Prize in Physics in 2002.

He graduated from the University of Tokyo in 1951 and received a Ph.D. in physics at the University of Rochester,
New York, in 1955. From July 1955 to February 1958 he was Research
Associate, Department of Physics, University of Chicago; from March 1958
to October 1963, he was Associate Professor, Institute of Nuclear
Study, University of Tokyo, although from November 1959 to August 1962
he was on leave from the above as Senior Research Associate with the
honorary rank of Associate Professor and as the Acting Director,
Laboratory of High Energy Physics and Cosmic Radiation, Department of
Physics, University of Chicago. At the University of Tokyo he became
Associate Professor in March 1963 and then Professor in March 1970 in
the Department of Physics, Faculty of Science, and Emeritus Professor
there in 1987. From 1987 to 1997, Koshiba taught at Tokai University. In 2002, he jointly won the Nobel Prize in Physics "for pioneering contributions to astrophysics, in particular for the detection of cosmic neutrinos". (The other shares of that year's Prize were awarded to Raymond Davis Jr. & Riccardo Giacconi of the U.S.A.)

He is now Senior Counselor of ICEPP and Emeritus Professor of University of Tokyo.

Koshiba's award-winning work centred on neutrinos, subatomic
particles that had long perplexed scientists. Since the 1920s it had
been suspected that the Sun shines because of nuclear fusion reactions
that transform hydrogen into helium and release energy. Later,
theoretical calculations indicated that countless neutrinos must be
released in these reactions and, consequently, that Earth must be
exposed to a constant flood of solar neutrinos. Because neutrinos
interact weakly with matter, however, only one in a trillion is stopped
on its way to Earth. Neutrinos thus developed a reputation as being
undetectable.

In the 1980s, Koshiba, drawing on the work done by Raymond Davis Jr,
constructed an underground neutrino detector in a zinc mine in Japan.
Called Kamiokande II, it was an enormous water tank surrounded by
electronic detectors to sense flashes of light produced when neutrinos
interacted with atomic nuclei in water molecules. Koshiba was able to
confirm Davis's results—that the Sun produces neutrinos and that fewer
neutrinos were found than had been expected (a deficit that became known
as the solar neutrino problem). In 1987 Kamiokande also detected neutrinos from a supernova explosion outside the Milky Way. After building a larger, more sensitive detector named Super-Kamiokande,
which became operational in 1996, Koshiba found strong evidence for
what scientists had already suspected—that neutrinos, of which three
types are known, change from one type into another in flight; this
resolves the solar neutrino problem, since early experiments could only
detect one type, not all three.

In 2003, he was awarded the Benjamin Franklin Medal in Physics.

Prof. Koshiba is a member of the Board of Sponsors of The Bulletin of the Atomic Scientists.